Make save_checkpoint more robust to non normal paths.

[copybara-service]

Make save_checkpoint more robust to non normal paths.

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[codecov-commenter]

Codecov Report

Merging #1446 (08f30bf) into main (543b48e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1446      +/-   ##
==========================================
- Coverage   82.29%   82.29%   -0.01%     
==========================================
  Files          65       65              
  Lines        5332     5331       -1     
==========================================
- Hits         4388     4387       -1     
  Misses        944      944              

Impacted Files	Coverage Δ
flax/training/checkpoints.py	98.36% <100.00%> (-0.02%)	⬇️

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